The present invention relates to metal pipe joints and, more particularly, to improved means for joining return bend connectors to heat exchange tubes of heat exchangers such as in refrigeration equipment.
Typically, refrigeration heat exchangers are made with U-bend coils arranged in serpentine fashion in a matrix of spaced metal heat transfer fins. Often, the coils are made of copper tubing. A refrigerant circulates through the tubes absorbing heat from air passed over the fins, thus cooling the air via convective heat transfer.
The joints between the tubular members of a heat exchange unit must provide a refrigerant-tight seal to ensure the integrity of the system. Commonly, the ends of the tubes being joined together are fit in concentric relation with one overlapping the other, in a telescopic fashion. A brazing material is used to fill the space between the overlapping ends. While it is desirable to employ relatively large clearances in the joint to facilitate the flow of brazing material, too large a gap makes it difficult to provide even penetration of the weld. Similarly, a minimum clearance is necessary to allow the brazing material to flow. Preferably, the clearance is such that the brazing material is pulled into the annular gap between the tubes by a capillary action. It is normally required that the resulting joint be as strong or stronger than the surrounding tubing to avoid providing a weak spot prone to structural failure. In addition, it is critically important to ensure the quality of the brazed joint to avoid subsequent corrosion and other problems resulting in leakage from the joint.
Several prior methods for soldering telescopic joints in serpentine tubing have involved centering the male member within the female member by providing outstanding nibs around the periphery of the male member. Such methods are shown in patents to DeCicco (U.S. Pat. No. 3,833,986), Watson, Jr. (U.S. Pat. No. 4,135,740), and Kilgore, et al. (U.S. Pat. No. 3,957,289). The primary focus of these prior methods has been to provide a frictional interference fit between the male member and the female member so that the U-shaped member on the end of the elongated tubes may be inverted and immersed into a solder bath. Watson and Kilgore both disclose axial grooves cut into the ends of the U-bends to allow the escape of hot gases from the solder process. These grooves can lead to irregularities in the circumferential quality of the soldered joint. The patent to DeCicco does not address the location or shape of the protruding nibs on the male member, the purpose of the nibs simply being to engage the interior of the female member in a frictional relationship.
The general brazing industry guideline has been that brazed joints should have axial "penetration equivalent to three times the thickness of the thinnest member being joined"--to meet strength and pressure requirements. Common practice is to exceed the minimum axial overlap to create a stronger joint under the general assumption that more is better. Additionally, prior techniques have not attempted to ensure the consistency of the amount of axial overlap.
Leakage of refrigerants has always been a concern for proper operation, safety and maintenance reasons, but has lately assumed more ominous, global implications. Refrigerants, once released, eventually disperse and rise into the ozonosphere. Widely used chlorofluorocarbon compounds, such as CFC-12 and R-512, break down under the sun's energy and release chlorine gas (Cl.sub.2). The freed chlorine molecules split and the chlorine atoms attach to the third oxygen atom in ozone (O.sub.3) forming ClO and O.sub.2. Recently, with the acknowledgment that this process measurably depletes the crucial ozone concentration of our planet's atmosphere, the search for refrigerant alternatives has intensified.
One such alternative, HCFC-22, long used in high-temperature cooling applications such as air-conditioning, has been promoted as the alternative refrigerant of choice to use on all new medium and low temperature applications in commercial refrigeration. Although switching to HCFC-22 at first glance alleviates environmental damage, serious problems have cropped up with the refrigerant leaking heavily from low-temperature systems. Losing large amounts of HCFC-22 into the atmosphere is, in itself, a major concern.
The present invention identifies and cures the cause of excessive leaking of alternative refrigerants from low-temperature systems by providing an improved pipe brazing structure which substantially eliminates the problems associated with prior art brazing techniques.